Technical Field
The present invention relates to gate stacks, and more particularly, to replacement gate stacks for p-type field-effect transistors and n-type field effect transistors and methods of forming the same.
Related Art
During integrated circuit fabrication, transistors can be formed in a semiconductor substrate. Each transistor includes a gate through which a current can be passed between source and drain terminals of the transistor. One approach to forming gates includes replacement metal gate (RMG) processing. RMG processing includes generating a number of sacrificial or dummy gates over a structure, such as a fin for a fin field effect transistor (FINFET), so that other processing steps, such as adjacent contact creation, can be carried out without damaging the gate. Once the other processing is carried out, the dummy gates are replaced with a metal to create the final metal gate structure. One challenge associated with RMG processing is reducing gate resistance in a p-type field-effect transistor without affecting the gate resistance in an n-type field-effect transistor or work function metals used to form the gates.
Generally speaking, a gate stack may include a wetting layer over a layer having a high dielectric constant (high-k layer) and an interfacial layer. Additionally, a gate electrode may be formed over the wetting layer. Titanium chloride (TiCl4) is used a precursor for low resistivity titanium nitride (TiN) wetting layers and tungsten (W) may be deposited thereover using a precursor of tungsten fluoride (WF6). Chlorine (Cl) and fluorine (F) each have been used to improve negative bias temperature instability (NBTI) by passivating defects in the bandgap of the high-k dielectric layer caused by dangling bonds, or unsatisfied valences. NBTI is a reliability issue which results in an increase in the threshold voltage and a decrease in both the drain current and transconductance of the integrated circuit when the NBTI is poor. However, chlorine (Cl) and fluorine (F) are not compatible to simultaneously improve the NBTI. The deposition of fluorine (F) from the tungsten fluoride (WF6) may penetrate through the titanium nitride (TiN) wetting layer to react with the high-k layer and the interfacial layer. However, this increases the thickness of the inversion layer, or the area under the gate, which in turn reduces capacitance. Therefore, a critical titanium nitride (TiN) wetting layer thickness is needed to prevent the increased thickness of the inversion layer. Conventionally, tungsten (W) deposition with tungsten fluoride (WF6) as a precursor, a nucleation layer and/or a cool fill tungsten (W) deposition is needed before chemical vapor deposition (CVD) of tungsten (W). Such nucleation layer and cool fill may offer higher resistivity than chemical vapor deposition (CVD) of tungsten (W) because of impurities that may be introduced due to the additional precursor needed for the nucleation layer and small tungsten (W) grain size due to the low temperature needed for cool fill tungsten (W).